Therapeutic drug monitoring (TDM) refers to the pharmacokinetic measurement of drug substance in the blood. TDM can be a powerful diagnostic tool for optimizing drug dosages on a patient-specific basis in cases where the / therapeutic window for a drug is narrow, and inter-patient variability is high. Cocktail therapies (HAART) used to treat HIV/AIDS suffer from high rates of treatment failure due to the dual limitations of drug resistance and chronic toxicity. These failures result from drug levels that are either too low or too high, respectively. To address the growing medical need for TDM in HIV/AIDS, we have designed a novel, enzyme-based assay (ENZ-PK) for measuring the total inhibitor concentration of HIV protease inhibitors (HIV PIs) in human blood samples. In preliminary studies, the ENZ-PK assay is fast, inexpensive, accurate and reliable. In this Phase I SBIR application, we propose to validate the practical clinical utility of the ENZ-PK assay to diagnose blood levels of PIs. In Specific Aim 1, we will measure the protease inhibitory potential from serum samples obtained from 300 patients treated with different PIs. Efforts will be made to ensure that clinical samples represent all six FDA-approved PIs. In Specific Aim 2, we will compare the ENZ-PK inhibitory potentials with PI drug concentrations measured using standard LC-MS methods. Protease inhibitory potentials will be reported in terms of the equivalent concentration of each PI An advantage that the ENZ-PK assay has over the LC-MS method is that it measures total concentration of all protease inhibitory substances in the blood, not just parent drug substance. This can be an important factor in cases where active metabolites are present; e.g., nelfinavir. The ultimate goal of this project is to adapt the ENZ-PK assay to a diagnostic kit format that can be used to provide routine clinical diagnostic labs with an easy, reliable and economical TDM assay that will give physicians rapid, one-day turnaround times. We anticipate that the demand for TDM of antiretroviral agents will become widespread as prospective clinical studies demonstrate positive benefits of TDM in conjunction with HAART.